Method for the
preparation of a catalysed particulate filter and catalysed particulate filter

ABSTRACT

A catalysed particulate filter coated with a wash coat on its dispersion side and its permeate side and within partition walls of the filter. The wash coat contains a first catalyst active in burning off soot, together with a second catalyst active in a selective catalytic reduction of nitrogen oxides.

This is a division of application Ser. No. 13/879,664, filed Apr. 16,2013, which is a 371 of PCT/EP2011/005504, filed Nov. 1, 2011, whichclaims priority to Danish Application No. PA 2010 00991, filed Nov. 2,2010.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to multifunctional catalysed dieselparticulate filters. In particular, the invention relates to a methodfor the preparation of catalysed diesel particulate filters having bothan activity in the removal of nitrogen oxides by the known selectivecatalytic reduction (SCR) process and an activity in the burning of soothaving been captured on the filter.

The invention provides furthermore a catalysed filter with a catalystbeing active in the SCR and a catalyst for improved burning of soot.

2. Description of the Related Art:

In addition to unburnt hydrocarbons, diesel exhaust contains, nitrogenoxides (NOx) and particulate matter. In particular NOx and particulatematter are material and chemical compounds representing a health andenvironmental risk and must be reduced or removed from the engineexhaust gas.

Particulate matter consists mainly of soot and is conventionally removedfrom the exhaust gas by passage through a filter arranged in the exhaustsystem of the engine.

Typically these filters are honeycomb wall flow filters, wherein thesoot is captured on or in partition walls of the honeycomb filter. Bytime, the build-up of captured particulate matter on the walls increasesthe pressure drop over the filter and the particulate matter must beremoved, usually by way of burning the soot. Burning off of the soot isusually accomplished passively in the presence of a soot combustioncatalyst or actively by a periodically increase of the temperature ofthe exhaust gas.

Filters provided with a catalyst catalysing a burning off of the sootare known in the art.

Beside a particulate filter, diesel exhaust gas cleaning systemsdisclosed in the art comprise a catalyst unit active in the selectivereduction of NOx by reaction with ammonia.

Multifunctional diesel particulate filters coated with catalystscatalysing different reactions are also known in the art.

In the known multifunctional filters the different catalysts aresegmentary or zone coated in different zones of the filter.

Segmentary or zone coating of different catalysts on the filter is,however, an expensive and difficult preparation process.

DESCRIPTION OF THE INVENTION

The present invention suggests a comparatively easier method for thepreparation of particulate filers catalysed with different catalysts forthe burning off of soot and a selective reduction of NOx.

Accordingly, the invention relates to a method for the preparation of acatalysed particulate filter comprising the steps of

a) providing a catalyst wash coat containing a first catalyst active inburning off of soot in combination with a second catalyst active inselective catalytic reduction of nitrogen oxides;

b) coating a particulate filter body with the catalyst wash coat on thedispersion side and the permeate side of the filter body and withinpartition walls of the filter body; and

c) drying and heat treating the coated filter body to obtain thecatalysed particulate filter.

The terms “dispersion side” and “permeate side” as used herein referboth to the flow passages of the filter facing the soot containingexhaust gas and to the flow passages facing the filtrated exhaust gas,respectively.

The main advantage of the method according to the invention is that thefilter can be coated with a single wash coat containing two types ofcatalyst catalysing different reactions. Thereby the preparation of amultifunctional catalysed filter is much improved in terms of afacilitated and cost-saving production setup. Additionally, the intimatemixture of the first and the second catalyst provides process advantageswhen the first catalyst presents some activity for the second reactionand the second catalyst presents some activity for the first reaction.This synergy increases the soot combustion activity and SCR activitycompared to a solution with only one of the catalysts. An example ofthis process advantage is demonstrated when the filter has a low sootloading because then the SCR NOx removal of the optimised SCR catalystis supplemented by the SCR activity of the soot combustion catalyst.Thus the appropriate catalyst is available in the filter where thereaction is needed, which is an improvement compared to a segmentarywash coated filters.

A further advantage of coating a filter with different types ofcatalysts as a mixture of catalyst particles is found in an improvedheat transfer and a warm-up during a cold start due to a low thermalmass of the combined filter catalyst brick and thus makes it possible tostart injection of a reductant and the SCR reaction earlier thanhitherto known.

In one embodiment of the invention, the first catalyst particles activein combustion of soot with oxygen or nitrogen dioxide may comprisecatalysts or catalyst precursors of one or more rare earth metal oxides.In addition to rare earth oxides, the first catalyst may furthercomprise zirconia, alumina, titania, silica, zeolite or combinationsthereof.

The first catalyst comprises preferably cerium oxide and zirconiumoxide.

In yet another embodiment of the invention, the second catalystparticles in the wash coat being active in the selective catalyticreduction of NOx may comprise at least one of a zeolite, a silicaaluminum phosphate, an ion exchanged zeolite, a silica aluminumphosphate optionally promoted with iron and/or copper, one or more basemetal oxides and a catalyst support of at least one of cerium tungstenoxide on a titanium oxide support, an alumina support, a zirconium oxidesupport or a silica support.

The second catalyst comprises preferably tungsten oxide and cerium oxidesupported on alumina.

In order to form the wash coat for use in the invention, the first andthe second catalyst usually in particle form are milled to the requiredparticle size and suspended in water or organic solvents, optionallywith addition of binders, viscosity improvers, foaming agents or otherprocessing aids.

The wash coat may be prepared by suspending the first and secondcatalyst particles in form of a single suspension or by preparing twodifferent suspensions with the first and the second catalyst,respectively, followed by mixing the two suspensions in a volume ratioto prepare the wash coat with the required amount of the first and thesecond catalyst particles.

Further binders, viscosity improvers, foaming agents may be added aftermixing.

The filter is then wash coated according to common practice.

In a preferred embodiment of the invention, the filter body is shaped inform of a wall flow monolith with a plurality of longitudinallyexpending passages formed by longitudinally extending walls bounding anddefining flow passages, where the dispersion side of the passages havean open inlet end and an outlet end being plugged with plugs, and wherethe permeate side of the passages have an inlet end being plugged withplugs and an open outlet end.

In yet another preferred embodiment, the filter body of the above wallflow monolith is wash coated before the outlet end of the dispersionside and the inlet end of the permeation side are plugged.

The invention provides furthermore a catalysed particulate filter coatedwith a wash coat on its dispersion side and its permeate side and withinpartition walls of the filter body, said wash coat containing a firstcatalyst being active in burning off of soot together with a secondcatalyst active in a selective catalytic reduction of nitrogen oxides.

A preferred filter body for use in the invention comprises a wall flowmonolith with a plurality of longitudinally extending passages formed bylongitudinally extending walls bounding and defining flow passages. Theinlet portion of the passages has an open inlet end and a closed outletend, and the outlet passages have a closed inlet end and an open outletend.

Examples of suitable filter materials for use in the invention aresilicon carbide, aluminium titanate, cordierite, alumina, mullite orcombinations thereof.

The amount of first catalyst on the filter is typically 10 to 100 g/land the amount of second catalyst on the filter is typically 20 to 180g/l. The total catalyst loading on the filter is typically in the range40 to 200 g/l.

The particle sizes of both catalysts in the wash coating suspension aresmaller than the mean pore size of the filter wall. But a further optionis the particle size of one of the catalysts in the wash coatingsuspension being equal to or larger than the mean pore size of thefilter wall.

EXAMPLE

A suspension of the first catalyst is in a first step prepared from amixture of cerium nitrate and zirconium nitrate followed by a heattreatment to convert the nitrate mixture into the oxides of cerium andzirconium with a molar ratio of Ce/Zr 3:1. In a second step, asuspension of the first catalysts is prepared by mixing 40 g of thecerium oxide zirconium oxide mixture in 80 ml demineralised water. Adispersing agent Zephrym PD-7000 and an antifoam agent are added. Thesuspension is milled in a bead mill. The particle sizes must be lowerthan the mean pore diameter of the pores in the wall of the wall flowfilter.

A suspension of the second catalyst is prepared by mixing cerium nitrateand ammonium meta tungstate solutions followed by impregnating the mixedsolution on aluminium oxide particles and heat treating the particles at550° C. for 4 hours in air. Thereby alumina particles with 10% Ceriumoxide and 10% tungsten oxide are obtained. 100 g of this tungsten oxideplus cerium oxide on aluminium oxide is dispersed in 200 mldemineralised water. A dispersing agent Zephrym PD-7000 and an antifoamagent is added. The suspension is milled in a bead mill. The particlesizes must be lower than the mean pore diameter of the pores in the wallof the wall flow filter.

A suspension of the first catalyst is then mixed into the suspension ofthe second catalyst.

A high porosity (approximately 60%) unplugged SiC wall flow filter isthen wash coated with the mixture of the first and the second catalystand dried.

After coating, the end channels are plugged with a commerciallyavailable cement and the plugged filter is subsequently heat treated at730° C.

What is claimed is:
 10. A catalysed particulate filter coated with awash coat on its dispersion side and its permeate side and withinpartition walls of the filter, said wash coat containing a firstcatalyst active in burning off of soot, together with a second catalystactive in a selective catalytic reduction of nitrogen oxides.
 11. Thecatalysed particulate filter of claim 10, wherein the filter is in formof a wall flow monolith with a plurality of longitudinally extendingpassages formed by longitudinally extending walls bounding and definingflow passages, the dispersion side of the passages have an open inletend and a closed outlet end, and the permeate side of the passages havea closed inlet end and an open outlet end.
 11. The catalysed particulatefilter of claim 10, wherein the first and second catalyst in the washcoat suspension have a particle size smaller than the mean pore size ofthe filter walls.
 12. The catalysed particulate filter of claim 10,wherein the first or the second catalyst in the wash coat suspension hasa particle size equal to or larger than the mean pore size of the filterwalls.